Bioequivalence is defined as the absence of a significant difference in the rate and extent to which the active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives becomes available at the site of drug action when administered at the same molar dose under similar experimental conditions in either a single dose or multiple doses in an appropriately designed study. If a drug is to be bioequivalent to the reference drug, the confidence interval for both pharmacokinetic parameters, AUC(area under the plasma concentration-time curve) and Cmax(maximal plasma concentration), must be entirely within the 80% to 125% of those of the reference drug. Underlying the concept of bioequivalence is the thesis that, if a drug product contains a drug substance that is chemically identical and is delivered to the site of action at the same rate and extent as another drug product, then it is equivalent and can be substituted for that drug product. The primary concern from the regulatory point of view is the protection of the patient against approval of products that are not bioequivalent. In this paper the general concept and the practical significance of the bioequivalence is described. The recently revised Korean guideline for bioequivalence test is also discussed.